1. Field of the Invention
The present invention relates to a gas laser having a vacuum-tight discharge tube, a laser capillary having at least a part of its length positioned in the interior of the discharge tube and in communication therewith. The gas laser includes an anode and a cathode disposed in the discharge tube and means for producing electrical discharge therebetween and in the laser capillary. The gas laser further includes two mirrors mounted on the discharge tube to form an optical resonator.
2. Prior Art
Gas lasers having a vacuum-tight discharge tube, a laser capillary with at least a portion of the capillary extending into the discharge tube and in communication therewith, having an anode and a cathode disposed in the tube with means for producing an electrical discharge therebetween and in the laser capillary and including two mirrors forming an optical resonator are known. In these known gas lasers, a discharge tube may have at least a portion consisting of a cylindrical tube of glass which has at least one of the ends closed off by a metallic plate which is provided with an opening behind which a mirror member is positioned on the outside of the discharge tube. For example, see Laser + Elektro-Optik, No. 4, 1974, page 64. In the case of this gas laser, which can be produced fully automatically and in relatively few production steps, the mirror members are directly fixed to the plates with a so-called "hard-seal" seal joint or sealing compound. It is true that such a sealing material has better sealing properties than the epoxy resin glues which are otherwise commonly used and particularly it appears to prevent diffusion of moisture into the discharge tube or tube enclosure; however, it does not provide the superior qualities of a permanently hermetically sealed glass solder closing.
Since it is not possible without difficulties to adapt the coefficient of thermal expansion of the mirror members, the closing plate and the glass tube to one another sufficiently and therefore the sealing must remain ductile, glass solder has not been considered for use in the tube construction of the just described prior art gas laser. When a metal part only serves as an end piece for a glass capillary, a much lesser critical geometry is given with respect to the thermal stresses, and for this reason the prior art has not tried to do more than solder the mirror member to such a metal part. For example, in U.S. Pat. No. 3,826,998, a mirror frame on which a mirror has been attached is used as an end section for a glass capillary. In the particular structure of this United States patent, the mirror frame includes a wall zone which has a weakened strength so that it may be permanently and plastically deformed by way of applying a tool from the outside of the capillary tube for purposes of adjusting the position of the mirror member.
In the prior art gas lasers, special electrical connections such as pins extending through the glass envelope are required. These pins in particular are for forming connections for the anode and cathode and cause an increase in the manufacturing cost and limit the use of automated assembly equipment.
With regard to the cathode, gas lasers usually use a cold cathode which is a seamless tube and which is held in a concentric spaced position within the tube forming the discharge tube. An example of such a structure is disclosed in U.S. Pat. No. 3,801,929. In the structure illustrated in this patent, the supports for mounting the seamless tube cathode in the discharge tube requires special production and mounting expenses. Furthermore, heat accumulation within the discharge tube may impair the life of the cathode and the structural stability of the discharge tube adjacent the cathode.
Instead of using a seam-free or seamless tube for the cathode, it has been suggested to use cathode tubes which are formed of a rolled sheet which has its edges folded into contact and into an interlocking joint. While such a cathode reduces some of the production costs, the problems with supporting the cathode and the heat dissipation problems still exist.
It has also been suggested to apply a layer or press a layer of aluminum onto an inner wall surface of the tube. While this construction improves the heat transport capabilities of the cathode and also decreases the non-active cathode surfaces which may release impurities into the atmosphere of the tube during operation, this type of structure increases the difficulty of construction during manufacturing.
If aluminum is used as the cathode material in a gas laser, normally a getter, such as zirconium-carbon getter, is inserted into the discharge tube in order to bind gases which are escaping from the surface of the aluminum cathode during operation of the laser. If zirconium were used as the cathode material instead of aluminum, a getter is not required; however, zirconium is a more expensive material. If the discharge tube is subjected to a baking-out process at a temperature of 400.degree. C or above, after the assembly of the discharge tube has been completed, it has been suggested that aluminum can be used as the cathode without requiring the addition of a getter. However, the thermal stresses that are created by the baking-out temperature, may cause detrimental effects on the structure of the presently known discharge tube.
The mounting of the laser capillary within the interior of a discharge tube presents several problems. It has been suggested to rigidly mount the laser capillary with respect to the discharge tube at one position and to support the capillary elastically such as by a spring element at at least one other position. Such a construction is disclosed in the German Offenlegungsschrift No. 2,129,142 dated Dec. 28, 1972. A support of a capillary tube using an elastic support for at least one position and a fixed or rigid support at a second position represents a compromise between a rigid two point securing or mounting of the capillary within the discharge tube and a single rigid mounting either at one end or at the center of the tube.
If the laser capillary is fused with the discharge tube at two positions, a mechanically rugged arrangement will be obtained. However, during the operation of the tube, stresses caused by the increased temperature may cause warping or bowing of the capillary and corresponding directional fluctuations which create intensity. losses in the output radiation of the gas laser. While this problem of stresses caused by an unbalanced thermal expansion during operation does not occur with the single fixed mounting, such as by fusing of the capillary to a portion of the discharge tube, the single fixed or rigid mounting of the capillary is very sensitive to shaking or vibration and thus is easily damaged while transporting the gas laser or while using the gas laser in an operation where it is subjected to vibrations.
The above-mentioned compromise of using an elastic mounting provided by a spring element along with a fixed or rigid mounting has experienced certaib difficulties. For example, the spring element must hold the capillary in a center position within the discharge tube with as little radial play as possible but at the same time with sufficient elasticity. Since the transverse dimensions of the capillary such as a glass tube and of the discharge tube usually vary, the previously used spring element usually had a relatively long spring path and a correspondingly weak spring force. Another suggested construction was to have the capillary extend into a counterbore in a massive metal end member of the discharge tube and the capillary was mounted in the bore by a slightly elastic cuff or bushing. Another problem with the spring elements is that spring elements made of steel have a decreased elasticity when heated to temperatures of several hundred degrees celsius (centigrade) such as 400.degree. C. Such a temperature is required for baking-out the assembled discharge tube in order to obtain a long life expectancy of the gas laser.